The present invention relates to an activated carbon and an evaporative fuel treatment apparatus using the activated carbon.
In a vehicle using gasoline as fuel, a canister (a carbon or charcoal canister) is normally used as the evaporative fuel treatment apparatus, in order to prevent evaporated fuel in a fuel tank from discharging into the atmosphere. The canister performs the function of adsorbing and desorbing the evaporated fuel as follows: The evaporated fuel generated from the fuel tank in an engine halt state is adsorbed by an adsorbent which is made of the activated carbon. After that, by negative pressure, generated by an intake of the engine, through the canister at engine start-up, an inside of the canister is purged with atmospheric air introduced from an air port. That is, the adsorbed evaporated fuel is desorbed from the adsorbent, and burnt in the engine. The adsorbent, therefore, regains its adsorbing capability by the purge, and thus being able to adsorb the evaporated fuel repeatedly and well.
Recently, in this kind of canister, an activated carbon having high adsorbing capability has tended to be used as the adsorbent, with miniaturization of the canister due to small layout of the vehicle, and also for request for increase in the adsorbing capability due to upsizing of the fuel tank. However, in a case where the activated carbon having high adsorbing capability is used as the adsorbent that fills the canister, once the evaporated fuel is adsorbed, it is difficult to desorb the adsorbed evaporated fuel from the adsorbent. That is, desorption efficiency is reduced, and there is a possibility that the inside of the canister may not be fully purged, even though the purge is done during travel. As a result, the so-called “blow-by phenomenon” might occur in which the adsorbed evaporated fuel, that remains in the canister without being purged, diffuses toward an adsorbent layer arranged at a side of the atmosphere in the canister, and the adsorbent can not fully adsorb the subsequent incoming evaporated fuel from the fuel tank, and the evaporated fuel without being adsorbed discharges into the atmosphere through the air port.
For the above problem, a canister has been disclosed in Japanese Patent Provisional Publication No. 2002-256989 (hereinafter is referred to as “JP2002-256989”). In JP2002-256989, the canister has three different types of activated carbons A, B, and C shown in FIG. 4, and therefore the canister prevents the blow-by phenomenon.
As seen in FIG. 4, the activated carbon A has characteristics that a peak is positioned at a pore diameter of the activated carbon, whose diameter satisfies an adsorption/desorption balance of vapor of gasoline, and curves on both sides of the peak gather around this pore diameter. The peak value of the pore diameter is substantially 2.5 nm, and an area (pore volume) formed by the curve of the activated carbon A and a horizontal axis is larger than that of the activated carbon B (described next). Here, the smaller the pore diameter, the greater the strength of adsorption (adsorbing force or power, or, holding force or power) tends to be. In addition, the area and an amount of adsorption are substantially proportional. Thus, the activated carbon A has characteristics that the holding power of the evaporated fuel is relatively weak, and the adsorption amount of the evaporated fuel is large.
As for the activated carbon B, the peak position is the same as that of the activated carbon A, but the pore volume is smaller than that of the activated carbon A. Thus, the activated carbon B has characteristics that the holding power of the evaporated fuel is relatively weak in the same manner as the activated carbon A, and the adsorption amount of the evaporated fuel is smaller than that of the activated carbon A.
As for the activated carbon C, the peak is positioned at a pore diameter of the activated carbon, whose diameter is suitable for the adsorption of low boiling point components (mainly, butane) in gasoline vapor, and curves on both sides of the peak gather around this pore diameter. Further, the peak position is around 2 nm, and the pore volume is small as compared with the activated carbons A, B. Thus, the activated carbon C has characteristics that the adsorption amount of the evaporated fuel is smaller than those of the activated carbons A, B, but the holding power of the evaporated fuel is relatively strong as compared with the activated carbons A, B.
In the canister of JP2002-256989, an adsorbent layer formed of the activated carbon A is arranged at a side of a charge port (tank port) and a side of a purge port, both of which are positioned at one end of an evaporative fuel passage formed in the canister. Further, an adsorbent layer formed of the activated carbon B, or two adsorbent layers: the adsorbent layer formed of the activated carbon B and an adsorbent layer formed of the activated carbon C, are arranged at a side of an air port which is positioned at another end of the evaporative fuel passage. The canister, therefore, prevents generation of the blow-by phenomenon.